Optical recording, a technique utilizing a focused laser beam to make micron size marks in an appropriate medium for high density information recording, has been extensively studied in recent years. There are basically two types of optical recording; write-once and the erasable. In write-once recording, the media can only be recorded once, but the recorded information can be read many times. In erasable recording, the recorded information can be erased and new information can be recorded over the same area of the media.
There are several commercially available write-once optical recording products, but the introduction of erasable products has been plagued with delays. One of the major difficulties has been the availability of good media.
The technique most widely studied for erasable recording has been based on magneto-optic materials. This technique relies on the thermal-magnetic recording process. A focused laser beam is used to heat a spot on a magneto-optical material so that its coercivity is reduced and the magnetization within the spot can be switched by an applied field. The readout is accomplished by sensing the Kerr rotation of a reading laser beam induced by the magnetization in the media. Good recording performance has been reported by many working in the field. However, all reports are based on rare-earth/transition metal alloys, notably TbFeCo. However, there are some disadvantages about magnetic-optical systems. First of all, the magneto-optical drive is more complex than an optical drive. Secondly, it takes great effort to make magnetic-optical medium directly over-writable. Another problem with the alloys is that the properties critical to the optical recording process are extremely sensitive to the composition of the alloys. A few percent deviation from the optimum composition can degrade the performance significantly.
An alternative technique for erasable recording uses amorphous-crystalline phase-change materials. In this technique, a focused laser beam is used to switch the material between the amorphous state and the crystalline state. As is commonly done, a high power laser is used to heat a spot on the material to above its melting point to randomize the atomic arrangement in the material. When the laser beam is switched off, the material is left in the metastable amorphous state because of the high cooling rate. A low power laser is then used to heat the material to below the melting point. The increased mobility of the atoms at the elevated temperature then allows the material to go to the more stable crystalline state. Thus by varying the power and duration of the laser beam, the material can be switched between the amorphous state and the crystalline state, and erasable recording is thus accomplished.
The major problem in the development of this technique has been the lack of appropriate materials. In particular, it has been difficult to find materials which have crystallization rate high enough under laser heating to allow high rate recording (erasure time &lt;1 .mu.s), and yet slow enough at room temperature to ensure data integrity.
With slower erasing materials, the erase beam spot is normally made elliptical. This means that two lasers are needed in the recorder head. With faster erasing materials, only one laser, providing a circular spot, is needed in the recording head. The simplicity and cost advantage of a one-laser head over a two-laser head is apparent. Also, lower power laser pulse means lower laser cost and shorter laser pulse means higher data rate. In addition, low power laser pulse is less likely to damage the substrate. It is evident from the above discussions that super-sensitive media will offer many advantages.
U.S. Pat. No. 4,787,077 describes a method of erasable recording using single-phase phase-change alloys. Whereas the crystallization rate of the preferred material, (GeTe).sub.0.85 Sn.sub.0.15, appeared to be high (erasure time &lt;55 ns), the laser power required for write and erase was also high (18 mW and 10 mW, respectively). While there was no mention of the corrosion resistance of the material, it contains a high concentration of corrosion prone tellurium.
Hiroshi Yasuoka et al. (Novel 1-Beam-Overwriting Method for Phase-Change Erasable Disc, Technical Digest, International Symposium on Optical Memory, Tokoyo, Japan (1987)), reported that In-Se-Tl can be cycled by 16 mW and 8 mW laser pulses of 60 ns. The problem with the composition of Yasuoka et al is that it requires higher laser power and longer laser pulse lengths than the present invention which, by comparison, requires laser powers less than 11 mW and a pulse length of 50 ns.
Tetsuya Nishida, et al. (Effect of Tl and Metallic Element Addition to In-Se based Phase-Change Optical Recording Film, Id., at 91) studied the same system, but they, too, failed to achieve the fast write-erase rates and high sensitivity of the present invention. Their write and erase pulses were 15 mW/60 ns and 12 mW/200 ns, respectively.
Noboru Yamada et al. (High Speed Over-writable Phase Change Optical Disc Materials, Id., at 87), investigated the pseudo-binary system of Sb2Te3-GeTe. A GeSb2Te4 thin film was cycled by 40 ns pulses at 25 mW and 12 mW. Although the pulse length was shorter in this pseudo-binary system, the laser power was higher than in the present invention.
Optical recording elements having Sb.sub.x Cd.sub.1-x or Sb.sub.x Cd.sub.y Sn.sub.z phase change alloy recording layers are made by depositing a layer of the alloy on an appropriate substrate. As deposited, the layers are amorphous. The amorphous as formed recording layer is first converted to a crystalline recording layer before use. This is commonly referred to as the initialization process.
A high power laser pulse can serve as the initialization pulse. However, the initialization can be accomplished in several ways such as by heating the material to above the melting point of the film using laser laser pulses of various duration, or by heating the films either in an oven or using high power short duration light pulses. The use of laser or light pulses is usually more desirable because these methods can heat up the media layer without affecting other components in the recording element.
As discussed above in describing the write-erase procedure, the use of super-sensitive media would offer many advantages. Thus, the problem that the present invention seeks to solve is to provide erasable optical recording media with improved sensitivity and fast write-erase rates.
The present invention also seeks to improve the corrosion resistance of the alloy.